Underwater off-gassing equivalent to a surface interval on air

[LFMarm]

Thanks all for the answers! I should probably rephrase the whole topic not as “optimal” but as “equivalent”. Depending on your deco gas, you may be off gassing in the water at certain depths as fast as you would at the surface on air. Which means that if you have enough gas/scrubber you may be able to stay in the water longer without affecting negatively your tissue gas loading.

 

[inquis]

Depending on your deco gas, you may be off gassing in the water at certain depths as fast as you would at the surface on air.

True enough at first glance. It does get complicated, though, at higher ppO2s. Gas transfer in the lungs becomes less efficient the longer you breath high ppO2 gas.

 

[LFMarm]

I don't see how your assertion holds for trimix, as helium is also an inert gas subject to absorption/deco considerations.

All the calculations I have done are just focused on ppN2. Are deco models incorporating He in the tissue loading?

 

[inquis]

Are deco models incorporating He in the tissue loading?

I know Buhlmann has separate sets of half-lives and M-values for helium and nitrogen.

 

[broncobowsher]

The really short, you run the highest PPO2 pratical until you reach a target PPO2 level. For basic CCR, 1.3 is normal. There are cahnges as you get more technical but for starters just run 1.3 PPO2.

When you get deeper than 1.3 PPO2 on near pure oxygen (Hard to get 100% in the loop, hard to get rid of that last little bit of inert stuff and not worth it) you add in inert gasses to keep the PPO2 at the level you want. Keep adding inert gasses (N2 and He in a fixed ratio based on the DIL fill) as you go deeper. Total partial pressures will add up to total pressure.

So it is completely possible, and happens quite often, your fast tissues will be void of normal level of N2 on the surface. Going off the loop and breathing air, you will be ongassing N2 on the surface

IF you have a Shearwater take a look at the tissue loading graph. Understand what the lines and color shifts mean. You will learn a lot of what is going on.

If you dive air dil, Get to a .79 PPN2. It will be the same nitrogen loading as breathing surface air. What depth does that happen at? Any that you get a .79 PPN2. Add the PPN2 and PPO2 numbers together, that will be the total pressure (depth) that you are at. .79 PPN2 and 1.21 PPO2 is 2.0 pressures, or about 30'. Drop some PPO2 numbers and you can still keep the PPN2 at a shallower depth.

There is no magic number depth where it always matches the surface, it is the pressure that says if it matches or not.

If you are at 0.2 PPN2 and 1.3 PPO2, you are hanging out in about 15' of water, degassing. And where the fast tissues loose almost all the dissolved Nitrogen.

 

[rx7diver]

All the calculations I have done are just focused on ppN2. Are deco models incorporating He in the tissue loading?

Yes. Keep in mind, having two (say) "inert" gases in the mix makes things tricky. Having only one "inert" gas makes the decompression calculations much easier. For example, N2 + O2 (EAN) or He + O2 (Heliox) for the entire dive, until switching to pure O2 for the 20 fsw decompression stop. Of course, you have to be careful of your oxygen exposure, though.

And you've got to eat a lot of blueberries and kale after your dive ...

rx7diver

 

[dmaziuk]

Isn’t a ppO2 higher than surface better for off-gassing?

No. The model is pretty simplistic, it only counts delta-pressure of inert gas(ses). Which is going to be greater topside so if you're OK with breathing pure O2, you're much better off doing it topside.

 

[rx7diver]

No. The model is pretty simplistic, it only counts delta-pressure of inert gas(ses). Which is going to be greater topside so if you're OK with breathing pure O2, you're much better off doing it topside.

Hmmm. Admittedly, I haven't done a decompression dive in nearly two decades, but back when I was doing them (Great Lakes extended range wreck diving), we were doing our last stop on pure O2 at 20 ffw. That is, we did NOT do a 10 ffw stop on O2; we would "deco out" completely at the 20 ffw stop. What you have written seems to be at odds with this.

Have things changed in this regard?

rx7diver

 

[dm9876]

Hmmm. Admittedly, I haven't done a decompression dive in nearly two decades, but back when I was doing them (Great Lakes extended range wreck diving), we were doing our last stop on pure O2 at 20 ffw. That is, we did NOT do a 10 ffw stop on O2; we would "deco out" completely at the 20 ffw stop. What you have written seems to be at odds with this.

Have things changed in this regard?

rx7diver

While "off-gassing" at 6m on O2 is happening at the exact same rate as it would at 3m or on the surface if still on O2 there are other reasons to stay lower. Some people like to avoid the swell if the surface is not dead flat but from a deco point of view it does limit the further growth of any bubbles that are present (which could themselves impede the off gassing process or cause DCS directly).

Dean

 

[dmaziuk]

Hmmm. Admittedly, I haven't done a decompression dive in nearly two decades, but back when I was doing them (Great Lakes extended range wreck diving), we were doing our last stop on pure O2 at 20 ffw. That is, we did NOT do a 10 ffw stop on O2; we would "deco out" completely at the 20 ffw stop. What you have written seems to be at odds with this.

Have things changed in this regard?

No, but if you take these two OP's statements: “Surface intervals are not always the most efficient way to off-gas nitrogen after a dive” and "optimal depth for off-gassing on O2 is 6m" together, at face value they spell "whaa?"

Obviously, if you have a planned deco stop on pure O2 at 6m, ignoring the plan and going up to 3m instead would not be optimal -- but not because off-gassing is somehow "worse" at shallower depth. And similarly, if you breathe O2 at 6 msw during your SI, it will speed up off-gassing -- but not because "SI is inefficient".